1. Field of the Invention
The invention relates to a diagnostic device and method for a fuel vapor treatment system provided in an internal combustion engine of a vehicle.
2. Description of the Related Art
In order to restrain the discharge of fuel vapor generated in a fuel tank into the atmosphere, an internal combustion engine mounted in a vehicle is provided with a fuel vapor treatment system that captures the fuel vapor in a canister. The fuel vapor treatment system is composed of a canister for capturing fuel vapor generated in the fuel tank, a purge passage for purging fuel vapor removed from the canister to an intake passage of the internal combustion engine, and a purge valve provided in the purge passage to adjust a flow rate of fuel vapor. In this fuel vapor treatment system, there is a limit to the amount of fuel vapor that may be retained by the canister. Therefore, a so-called purge treatment is carried out. That is the purge valve is opened to remove fuel vapor from the canister during the operation of the engine, and the removed fuel vapor is introduced into the intake passage via the purge passage to be burned in a combustion chamber By executing the purge treatment, the fuel vapor adsorption performance of the canister is recovered.
In the fuel vapor treatment system, when a hole is formed or a sealing failure occurs in a fuel vapor path that extends from the fuel tank to the purge valve via the canister, fuel vapor leaks into the atmosphere. As a result, the fuel vapor treatment system cannot sufficiently perform its intended function.
Thus, various devices for diagnosing the leak of fuel vapor from the fuel vapor path are proposed. For example, in the device described in Japanese Patent Application Publication No. 2002-180916 (JP-A-2002-180916), a pressure in an fuel vapor path is increased by a pump, and the leak of fuel vapor is diagnosed based on a diagnostic parameter that changes depending on whether fuel vapor leaks at the time of pressurization. Further, in the device, to restrain fuel from leaking through a feed oil port as a result of the increase in the surface of oil in a fuel tank when the pressure in the fuel vapor path is increased, permission to execute the leak diagnostic is determined based on the angle of inclination of the vehicle.
If the leak diagnostic as described in Japanese Patent Application Publication No. 2002-180916 (JP-A-2002-180916) is carried out when fuel vapor actually leaks from the fuel vapor path, the occurrence of a leak is detected based on the described diagnostic parameteL However, because the pressure in the fuel vapor path is increased while the leak diagnostic is carried out, there is a possibility that a larger amount of fuel vapor may be discharged into the atmosphere through the leak.
Thus, when the pump is used not to increase but to reduce the pressure in the fuel vapor path in carrying out the leak diagnostic, the pressure in the fuel vapor path is below a pressure outside the fuel vapor path, namely, the atmospheric pressure. Therefore, fuel vapor leaks from the fuel vapor path during the leak diagnostic may be reduced.
However, if the pressure in the fuel vapor path is reduced using the pump as described above, then when the vehicle is inclined during the leak diagnostic, liquid fuel in the fuel tank may be drawn in by the pump and delivered into the canister or the pump itself. If liquid fuel is thus drawn into the canister, the capture performance of the canister, which is provided to adsorb fuel vapor, deteriorates, and liquid fuel may be introduced into the intake passage due to the purge treatment in some cases. Further, the pump for pressure reduction is also intrinsically designed to draw in gaseous fuel. Therefore, if liquid fuel flows into the pump, a deterioration in the performance thereof or a malfunction thereof may be caused.
It is also conceivable to form the fuel tank and the like in such a shape that liquid fuel may be restrained from being drawn in by the pump. In this case, however, there is a limit to the angle of inclination of a vehicle that can be coped with. It is difficult to cope with an excessively large angle of inclination.